The reliability of a system based on the computed fade margin is calculated based on the following equation (from Lenkurt)

Undp = a x b x 2.5 x 10-6 x f x D3 x 10-F/10

where:

• Undp is the non-diversity outage probability.
• a is the terrain factor.
• b is the climate factor.
• f is the frequency in GHz.
• D is the path length in miles.
• F is the fade margin in dB.

For systems which include a space antenna, a fade margin is computed for each receive antenna (primary and diversity). The values will be different if the two receive systems have different antenna gains, transmission line lengths, or other losses. The higher value fade margin (“best case”) is used for this non- outage calculation. The other fade margin value is used to compute the diversity as described below.

The percent reliability is computed from the outage probability by:

%R = 100 x (1 - Undp)

In the Two-Way Microwave Link Budget, reliability is assessed in each direction.  In the Undp calculation, the terrain and climate factors are defined differently, but the equation is otherwise identical (from Kizer)

Undp = (Ts / Tref) x C x (50 / W)1.3 x 2.5 x 10-6 x f x D3 x 10-F/10

where:

• Undp is the probability of non-diversity flat fading outage during heavy fading.
• T_s is the number of seconds in the heavy fading season = 8 x 10⁶ x T/50.
• T_ref is the number of seconds in a year.
• T is the average annual temperature, in degrees Fahrenheit; 35 < T < 75.
• W is the terrain roughness, in feet, limited to the range [20, 140].
• C is the climate and humidity factor (X in the TAP dialog).
• f is the frequency in GHz.
• D is the path length in miles.
• F is the fade margin in dB.

Reference:

Engineering Considerations for Microwave Communications Systems, 1975, GTE Lenkurt Incorporated.

Kizer, George (2013). Digital Microwave Communication: Engineering Point-to-Point Microwave. Wiley.

ANSI/TIA-10-A-2023. TIA Standard: Interference Criteria for Microwave Systems.  April 2023.